Use AVFoundation framework to support older macOS versions

[clearlysid]

We are currently using Apple's ScreenCaptureKit APIs, which are quite modern, performant and secure. However it is only present in macOS 12.3+. To support older macOS versions, we should extend our engine to use AVFoundation Framework.

Wulkano's Aperture project uses this framework, here is the relevant swift code. I've also previously done some work to bring Aperture to Rust via aperture-rs, but I think it will be better to just call the relevant APIs and manage the abstraction from within scap itself.

This is a non-urgent feature request from my side, but open to prioritising it higher if there's more community interest.